1. Field of the Invention
This invention relates to a semiconductor device and a method of manufacturing the same and more particularly to a semiconductor device having a silicide film and a method of manufacturing the same.
2. Description of the Related Art
As the performance of a MOS transistor is more enhanced, it becomes more popular to form the gate electrode in a silicide form in order to reduce the parasitic resistive component thereof. For an integrated circuit such as an SRAM which requires extremely high integration density, a transistor structure in which the gates of an NMOS transistor and a PMOS transistor are used as one Si gate pattern and a junction portion between an N+ diffusion layer and P+ diffusion layer in the Si gate pattern is short-circuited by use of a silicide film is formed.
In a case where the N+ diffusion layer and P+ diffusion layer are thus formed in the same Si gate pattern, normally, the Si gate pattern is formed with a resist mask and N-type and P-type impurities are selectively ion-implanted. At this time, the N+ diffusion layer and P+ diffusion layer may be superposed depending on the alignment position of the resist mask and an impurity mixture region in which N-type and P-type impurities exist in a mixed form may be formed in the Si gate pattern in some cases. The thickness of a natural oxide film formed on the surface of the impurity mixture region is different from the thickness of a natural oxide film formed on the surface of the N+ diffusion layer and the thickness of a natural oxide film formed on the surface of the P+ diffusion layer.
Further, it is known that the natural oxide film formed on the surface of the P+ diffusion layer is more difficult to remove than the natural oxide film formed on the surface of the N+ diffusion layer. More specifically, since the concentration of holes in the natural oxide film or oxide film formed on the surface of the P+ diffusion layer becomes higher, it is difficult to completely remove the oxide film.
Reference document: Sato et al. “Study of HF-Treated Heavily-Doped Si Surface Using Contact Angle Measurements” Jpn. J. Appl. Phys. Vol. 33 (1994), pp 6508 to 6513.
When a silicide film is formed on the surface of the Si gate pattern, a step of removing the natural oxide film from the surface of the Si gate pattern is provided as the preprocessing step. However, if the thickness of the natural oxide film formed on the surface of the Si gate pattern varies and the difficulty in removing the natural oxide film varies, the natural oxide film cannot be completely removed in the preprocessing step and may be partly left behind on the surface of the Si gate pattern in some cases. The thus remaining region of the natural oxide film will obstruct the silicidation reaction between Si and metal. As a result, the resistance may increase in the remaining region of the natural oxide film in the Si gate pattern and an “open” defect may occur. Next, an example of the problem is explained.
FIGS. 33A to 33E are cross sectional views showing a manufacturing method of the conventional semiconductor device in the order of the manufacturing steps and particularly showing a case wherein an impurity mixture region is formed in the Si gate pattern.
First, as shown in FIG. 33A, a P+ diffusion layer 104, N+ diffusion layer 105 and N+/P+ mixed layer 107 are formed in an Si gate pattern 101. Further, a natural oxide film 110 is formed on the surface of the Si gate pattern 101 and, particularly, the film thickness t107 of the natural oxide film formed on the surface of the impurity mixture region 107 is different from the film thickness t104 of the natural oxide film formed on the surface of the P+ diffusion layer 104 and the film thickness t105 of the natural oxide film formed on the surface of the N+ diffusion layer. Specifically, the film thickness t107 is larger than the film thickness t104 and the film thickness t107.
Next, as shown in FIG. 33B, the natural oxide film 110 is etched by a wet etching process using hydrofluoric acid or the like. At this time, it is assumed that the natural oxide film 110 is partly left behind on the surface of the impurity mixture region 107.
Then, as shown in FIG. 33C, a metal film 111 is formed on the Si gate pattern 101 with the natural oxide film 110 partly left behind thereon.
After this, as shown in FIG. 33D, the heat treatment is performed to cause a reaction between the Si gate pattern 101 and the metal film 111 so as to form a silicide film 109. At this time, since the reaction is difficult to occur on the natural oxide film 110, the silicide film 109 is not practically formed on the natural oxide film 110.
Next, as shown in FIG. 33E, a non-reacted portion of the metal film 111 is removed. Thus, the Si gate pattern 101 whose surface is formed in a silicide form can be obtained.
However, since the silicide film 109 is not practically formed on the impurity mixture region 107, the silicide film 109 is divided on a boundary portion 106 between the P+ diffusion layer 104 and the N+ diffusion layer 105. As a result, a junction portion between the P+ diffusion layer 104 and the N+ diffusion layer 105 cannot be short-circuited by use of the silicide film 109. For example, this may be a cause of the “open” defect.
FIGS. 34A to 34E are cross sectional views showing another manufacturing method of the conventional semiconductor device in the order of the manufacturing steps and particularly showing a case wherein a natural oxide film is left behind on the surface of a P+ diffusion layer.
First, as shown in FIG. 34A, a P+ diffusion layer 104 and N+ diffusion layer 105 are formed in an Si gate pattern 101 and a natural oxide film 110 is formed on the surface of the Si gate pattern 101.
Next, as shown in FIG. 34B, the natural oxide film 110 is etched by a wet etching process using hydrofluoric acid or the like. At this time, it is assumed that the natural oxide film 110 is partly left behind on the surface of the P+ diffusion layer 104.
Then, as shown in FIG. 34C, a metal film 111 is formed on the Si gate pattern 101 with the natural oxide film 110 partly left behind thereon.
After this, as shown in FIG. 34D, the heat treatment is performed to cause a reaction between the Si gate pattern 101 and the metal film 111 so as to form a silicide film 109. At this time, as explained with reference to FIG. 33D, the silicide film 109 is not practically formed on the natural oxide film 110.
Next, as shown in FIG. 34E, a non-reacted portion of the metal film 111 is removed. Thus, the Si gate pattern 101 whose surface is formed in a silicide form can be obtained.
However, since the silicide film 109 is not practically formed on the natural oxide film 110, the silicide film 109 is divided on the P+ diffusion layer 104. As a result, the resistance will increase on a region of the P+ diffusion layer 104 on which the natural oxide film 110 is left behind.
The problems caused by leaving behind the natural oxide film 10 on the Si gate pattern 101 can be solved by increasing an etching amount of the natural oxide film 110 in the steps shown in FIGS. 33B and 34B, for example. However, if an etching amount of the natural oxide film 110 is increased, excessive etching will occur in a portion of the integrated circuit, for example, in the element isolation region. Next, a typical example of a problem caused by the excessive etching is explained below.
FIGS. 35 to 39 are cross sectional views showing a manufacturing method of the conventional semiconductor device in the order of the manufacturing steps and particularly showing a salicide process.
First, as shown in FIG. 35, an element isolation region 122 is formed on the surface region of an N-type well region 121 to define an element region 123. In the element region 123, P+ diffusion layers 124 and P− diffusion layers 125 which act as the source/drain regions of an MOSFET are formed. The P− diffusion layer 125 is a region called an LDD (Lightly Doped Drain) region or extension region in the MOSFET with the LDD structure. A gate insulating film 126 is formed on a channel region between the P− diffusion layers 125 and a gate electrode 127 is formed on the gate insulating film 126. The gate electrode 127 is formed of silicon having P-type impurity doped therein and, for example, corresponds to the P+ diffusion layer 104 of the Si gate pattern 101 shown in FIGS. 33A and 34A. A side wall insulating film 128 is formed on the side walls of the gate electrode 127 and on the P− diffusion layers 125. The side wall insulating film 128 is a silicon oxide film. A natural oxide film 110 is formed on the surfaces of the P+ diffusion layers 124 and on the surface of the gate electrode 127.
Next, as shown in FIG. 36, the natural oxide film 110 is etched by a wet etching process using hydrofluoric acid or the like. In the etching process, it is assumed that an etching amount of the natural oxide film 110 is increased to completely remove the natural oxide film.110 from the surface of the gate electrode 127 and the surfaces of the P+ diffusion layers 124. At this time, excessive etching occurs in the element isolation region 122 and side wall insulating film 128 to reduce the film thicknesses thereof. In this case, if the film thickness of the element isolation region 122 is extremely reduced, the upper surface of the etched element isolation region 122 becomes lower than the junction between the P+ diffusion layer 124 and the N-type well region 121 as indicated by a reference symbol 130 so as to expose a portion of the N-type well region 121 in some cases.
Then, as shown in FIG. 37, a metal film 111 is formed on the structure in which the portion of the N-type well region 121 is exposed.
After this, as shown in FIG. 38, the heat treatment is performed to cause a reaction between Si of the gate electrode 127 and element region 123 and the metal film 111 so as to form a silicide film 109.
Next, as shown in FIG. 39, a non-reacted portion of the metal film 111 is removed. Thus, the surface of the gate electrode 127 and the surface of the P+ diffusion layer 124 are formed in a silicide form.
However, since the film thickness of the element isolation region 122 is reduced and the N-type well region 121 is partly exposed, the silicide film 109 is formed to extend over the P+ diffusion layer 124 and N-type well region 121. As a result, the P+ diffusion layer 124 and the N-type well region 121 are short-circuited via the silicide film 109. Then, a problem that a junction leak between the P+ diffusion layer 124 and the N-type well region 121 is increased or the MOSFET will not be operated will occur.